Currently, in systems operating according to the specifications for Long Term Evolution (LTE) systems developed by the 3rd-Generation Partnership Project (3GPP), a wireless terminal in RRC_CONNECTED state is required to perform measurements on the physical layer. These measurements are used by the physical layer to determine whether the air interface is properly synchronized and whether the radio link is properly established, and to report the physical layer status to higher layers of the terminal's protocol stack. The higher layers need this information to communicate reliably with the eNodeB (3GPP terminology for a base station).
Typically, the wireless terminal does the measuring by listening to cell-specific reference signals (CRS) transmitted by the eNodeB and measuring the signal strength. The exact measurement requirements are tailored to work with different Discontinuous Reception (DRX) cycle lengths, so that the time period over which the measurements are done depend on the DRX cycle length and status. Details are provided in the 3GPP specification “Requirements for support of radio resource management,” 3GPP TS 36.133, v. 11.1.0 (July 2012), available at www.3gpp.org. Similarly, when no DRX is used, there are requirements to measure the signal quality over a time period that is typically shorter than when DRX is used, where the exact length of the time period depends on whether the terminal (“user equipment,” or “UE,” in 3GPP terminology) is detecting out-of-synch or in-synch events.
Depending on these measurements and on preset threshold levels, the physical layer of the wireless terminal indicates to upper layers whenever it becomes out-of-synch or in-synch, depending on its earlier status. When a certain number of out-of-synch messages is received by the Radio Resource Control (RRC) layer, a timer is started and upon the expiration of this timer a radio link failure (RLF) is declared. (The RRC protocol for LTE systems is defined in “Radio Resource Control (RRC); Protocol Specification,” 3GPP TS 36.331, v. 11.1.0 (September 2012), available at www.3gpp.org.) After RLF, the radio connection is typically either re-established or the terminal is moved to RRC_IDLE state.
Machine-to-machine (M2M) communication, sometimes referred to as machine-type-communication (MTC), is an increasingly popular paradigm, and M2M traffic and devices are envisioned to grow to huge numbers in the near future. For M2M communications, different optimizations to current technologies are needed, depending on the use case. One goal is to ensure the lowest possible power consumption, meaning that extensive or unnecessary signaling should be avoided and that the device should be kept in low-power state for long periods (i.e., having as few transmission and reception events as possible). In a typical implementation, a device, once not needed to receive or transmit data, goes into a deep sleep mode where as much as possible of the device's circuitry is turned off. In deep sleep, the power consumption can be on the order of a fraction of percent compared to high-power active transmission/reception mode. However, in order to reliably wake up at a certain time, the device needs to have some sort of low-power clock powered on. However, low power clocks are often inaccurate; hence, there is a risk of slipping in time and frequency during the deep sleep.
According to current specifications, measurements taken to detect an out-of-sync condition are spread over several DRX cycles. To minimize power consumption in an M2M device, it is desirable to lengthen the DRX cyte times. However, if the DRX cycle length is increased and is considerably longer than the current maximum, e.g., in excess of two minutes, and if the radio measurements discussed above are done over many DRX cycle lengths, as currently specified, then the device's synchronization status and actual radio link failure cannot be detected in a timely manner. The number of snapshots used for a reliable measurement is already quite small, i.e., on the order of five snapshots, with each snapshot being taken over a short time interval on the order of a few subframes (2-5 milliseconds). Using fewer snapshots for a measurement will produce unreliable measurements, e.g., due to fading. As a result, simply reducing the number of snapshots so that a measurement is taken over a reduced number of DRX cycle lengths would not work well.
Accordingly, improved solutions are needed for handling measurements in the context of extended DRX cycle lengths.